This application is based on Patent application No. 11-22403 filed on Jan. 29, 1999 in Japan, the content of which is incorporated herein to by reference.
1. Field of the Invention
The present invention primarily relates to a solder alloy mainly used in metal bonding of an electric device. In particular, the present invention relates to a lead free, eco-friendly solder alloy that does not cause pollution.
2. Description of Related Art
In an electric device, when solder bonding is performed, a solder alloy is required to have a desired bonding temperature, proper wettability during bonding, and superior thermal fatigue strength, ductility, and heat resistance. In addition, such a solder alloy is desired to be free of lead in consideration of environment conservation.
Conventional solder alloys include Snxe2x80x94Pb, Snxe2x80x94Ag, Snxe2x80x94Sb, and Snxe2x80x94Bi alloys.
A typical Snxe2x80x94Pb alloy, 63Sn-37Pb (which means 63 wt.% Sn- 37 wt.% Pb, and has an eutectic point of 183xc2x0 C.) contains lead. Thus, such Snxe2x80x94Pb alloy may cause lead pollution, and is not eco-friendly. Lead-free solder alloys which may be substituted for 63Sn-37Pb include an Snxe2x80x94Ag alloy (Sn-3.5 Ag, eutectic point of 221xc2x0 C.) and an Snxe2x80x94Sb alloy (Sn-9 Sb and peritectic temperature of 246xc2x0 C.). These alloys have higher melting points than the 63Sn-37Pb eutectic solder, and have a problem with heat resistance during bonding for some electronic parts. Thus, a solder alloy having a melting point lower than these melting points is desired. A method for lowering a melting point includes an addition of Bi or In to Sn. A 42Sn-58Bi alloy has an eutectic point of 139xc2x0 C., and a 52Sn-48In alloy has a eutectic point of 117xc2x0 C.
As a lead-free alloy with a low melting point, a Snxe2x80x94Bi based alloy having a composition of Sn-7.5 Bi-2.0 Ag-0.5 Cu has been disclosed in Japanese Patent Application Laid-Open No. 8-206874 and Scripta Materialia, Vol. 38, No. 9, pp. 1333-1340, 1998. However, lead-free Sn-based alloys have poorer wettability and bonding properties than Snxe2x80x94Pb alloys, since Sn is prone to oxidation. Further, Bi and In that are additive elements for lowering the melting point of lead-free Sn-based solder alloys are more prone to be oxidized than Sn. Thus, there has been a problem that the above mentioned Snxe2x80x94Bi based alloys and Snxe2x80x94In alloys have unstable solderability. In addition, when Bi is added, it tends to become hard and brittle. Thus, there has been another problem that the above mentioned alloys have lower thermal fatigue strength.
The present invention has been achieved to solve the above mentioned problems. It is an object of the present invention to provide a novel lead-free Snxe2x80x94Bi based alloy having improved wettability and heat resistance as compared to conventional Snxe2x80x94Bi based alloys, a lower melting point than the eutectic point of 221xc2x0 C. of the Snxe2x80x94Ag alloy, and proper solderability.
According to a first aspect of the present invention, the above mentioned object is achieved by providing a solder alloy containing tin, as a major component, and 21 wt.% or less bismuth, 4 wt.% or less silver, 2 wt.% or less (inclusive of zero) copper, and 0.2 wt.% or less nickel.
According to a second aspect of the present invention, the above object is achieved by providing a solder alloy containing tin, as a major component, and 21 wt.% or less bismuth, 4 wt.% or less silver, 2 wt.% or less (inclusive of zero) copper, and 0.1 wt.% or less germanium.
According to a third aspect of the present invention, the above object is achieved by providing a solder alloy containing tin, as a major component, and 21 wt.% or less bismuth, 4 wt.% or less silver, 2 wt.% or less (inclusive of zero) copper, 0.2 wt.% or less nickel, and 0.1 wt.% or less germanium.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.